According to Wikipedia, it was published in 1927, so it might still be in copyright. I cannot find a record for it on
www.copyright.gov. It was recently rereleased, at the same time as the movie. This gives us a paper copy to check for a copyright notice.
P.S. But that is US law. The work of Upton Sinclair cannot be posted to this forum because the forum is hosted in Canada. Mr.Sinclair died in 1968, which is less than 50 years ago. Whatever we have will have to be removed (at least temporarily).